Nanometric Surface Finish Research Articles

Flow field analysis of the thin fluid film in disc hydrodynamic polishing

Nowadays, large aspheric surfaces, including non-rotationally-symmetric surfaces, are increasingly used in ground and space-based astronomical instruments. The fabrication of these surfaces with sub-micrometric form accuracy and nanometric surface finish, especially for hard and difficult-to-machine materials, has always been a challenge to the optics industry. To efficiently produce ultra-smooth surfaces without the subsurface damage and surface scratches, a novel Disc Hydrodynamic Polishing (DHDP) is proposed by the combination of Elastic Emission Machining (EEM) and Fluid Jet Polishing (FJP). Firstly, the polishing tool for DHDP was carefully designed and the feasibility of the proposed method was experimentally verified. It is interesting to find that the liquid film acts as a carrier of abrasive grains between the polishing tool and polished surface. Then, computational fluid dynamics (CFD) was used to study the effects of process parameters on the slurry film flow in disc hydrodynamic polishing. Finally, preliminary experiments are conducted to verify the CFD simulations. The experimental data reasonably agree with the simulation results. The results also show that increasing rotation speed has no influence on the film thickness for polishing tool without grooves, but leads to increased film thickness for polishing tool with grooves. The film thickness increases as the inlet pressure is increased for these two polishing tools.

Micro Electro Discharge Machining of Non-Conductive Ceramic

Electro discharge machining (EDM) as a die sinking process has taken off in 1943. Since then it was known as a non-conventional machining process and its application was limited for processing only electrically conductive materials. Later on, due to the widespread applications, this EDM process is considered conventional as usual. However, in the recent years EDM has gone through considerable changes especially with dielectric fluids, simple to complex geometry, meso to micro sized structures, nanometric surface finish, and so on. In addition, the application of EDM has also been extended for processing electrically semi-conductive and non-conductive materials like ceramics and composites. This paper discussed micro electro discharge machining of non-conductive ceramic materials. It includes detail process development, modelling of material removal rate and surface finish which include the effect of multi spark and random spalling conditions.

Experimental and simulation study of nanometric surface roughness generated during Magnetorheological finishing of Silicon

Silicon Mirrors are essential components for guiding the X-Ray beam and focusing it to a particular location. These mirrors using total internal reflection require super smooth surface finish because X-Ray wavelength is very small, otherwise it suffers from strong scattering. The polishing possibility of silicon mirrors by Magnetorheological finishing process is examined. Magnetorheological finishing (MRF) is a computer-controlled technique that is used in the production of high quality optical lenses. This novel finishing process utilizes polishing slurries based on magneto rheological fluids, whose viscosity changes with the change in magnetic field. In this study, the MRF is used to polish the silicon mirror in order to achieve nanometric surface finish to be used for X-rays applications. The individual effect of parameters like magnetizing current, working gap, rotational speed on surface roughness is investigated. Based on the experimentation, the optimized process parameters are identified. The final surface roughness achieved is as low as 6.4nm. The surface quality is analyzed in terms of arithmetic roughness (Ra) and Scanning Electron Microscopy for uniform evaluation. In order to investigate the physical essence underlying MRF process, the Molecular Dynamics Simulations (MDS) is used. MDS is used to study the atomic-scale removal mechanism of single crystalline silicon in Magneto Rheological Finishing (MRF) process and particular attention is paid to study the effect of gap between the tool and work piece as well as cutting velocity on surface quality.

Review on fast tool servo machining of optical freeform surfaces

Fast tool servo (FTS) in ultra-precision machining (UPM) is an enabling and efficient technology for fabricating optical freeform surfaces or microstructures with submicrometric form accuracy and nanometric surface finish. There are many kinds of FTS in the different driving principle to present their various performances currently. Their kernel technologies influence the machining ability and accuracy of freeform surfaces, consequently receiving much research attention and interest. These technologies are generally summarized as the development of FTS structure, the advanced control algorithms, tool path planning, machining condition monitoring, and surface measurement and error compensation. This paper aims to survey the current state of the art in machining freeform optics by FTS. An analysis of the principle, performance, and application of FTS machining with regard to freeform optics is presented. And the key machining technologies for optical freeform surfaces by FTS are then introduced in detail. The challenges and opportunities for further studies are concluded according to the FTS machining difficult of optical freeform surfaces finally.

An Experimental Investigation of Effect of Process Parameters on Materials Removal Characteristics in Fluid Jet Polishing

Fluid Jet Polishing (FJP) is a promising ultra-precision machining technology which shows great potential with regard to the application value in the generation of freeform surfaces with sub-micrometre form accuracy and nanometric surface finish. The polishing tool influence function (TIF), which is affected by various parameters and commonly assessed in terms of width, maximum depth and volumetric material removal rate, is critically required for corrective polishing and deterministic machining. In this study, a series of experiments was conducted to study the effect of various process parameters on the polishing tool influence function. The polishing machine used is a Zeeko IRP 200 ultra-precision freeform polishing machine, with three linear axes and three rotational axes. All polishing experiments were performed on BK7, which is one of the most common technical optical glass materials for high-quality optical components in the visible range. These specimens were polished using Al2O3 (Aluminium oxide) abrasive particles and measured by a Zygo Nexview 3D Optical Surface Profiler. All these experiments were conducted by changing one process parameter and keeping the other process parameters constant. In this paper, the experimental results are described in detail to show the relationships between the material removal and the various parameters (i.e. slurry pressure, standoff distance, impingement angle, etc.), and some interesting experimental results are explained.

Form Accuracy of Optical Mould Inserts Made from Rapidly Solidified Aluminium Alloys

Plastic optical components and lenses produced in mass quantities are usually manufactured using high-precision plastic injection technology. For that, high-precision plastic moulds with aluminium optical inserts made with extremely high dimension accuracy and high optical surface quality are used. Ultra-high precision single-point diamond turning have been successfully used in shaping optical mould inserts from various aluminium grades such as traditional 6061. However, extreme care should be taking when selecting machining parameters in order to produce optically valid surfaces before premature tool wear takes place especially when the machined optical materials has inadequate machining database. The current experimental study looks at the effect of cutting conditions on optical surfaces made from aluminium. The study embarks on helping establish some diamond machining database that helps engineers select the most favourable cutting parameters. The papers reports on the accuracy and surface finish quality received on an optical surface made on mould inserts from a newly developed aluminium alloy. Rapidly solidified aluminium (RSA) grades have been developed recently to address the various problems encountered when being cut by single-point diamond turning operation. The material is characterised by its extremely fine grained microstructure which helps extend the tool life and produce optical surfaces with nanometric surface finish. It is found the RSA grades can be successfully used to replace traditional optical aluminium grades when making optical surfaces. Surface finishes of as low as 10 nanometres and form accuracy of less than one micron can be achieved on RSA.

Theoretical Analysis about Off-Axis Parabolic Surface Machining under Two Different Accurate Spiral Tool Paths

Off-axis aspheric surface is an indispensable optical device for the high precision measurement and manufacturing equipment, and the tool path generation is a very important factor for the manufacturing to obtain the surface with high precision form and nanometric surface finish. In this paper, the systemic description of tool path generation method about the off-axis parabolic surface with ultra-precision single point diamond turning is processed. Two effective methods of generating the accurate spiral tool path are proposed and compared. The proposed methods are mainly based on mathematical calculation directly from analytic surfaces, including the general process of tool path generation, cutting-tool compensation and avoiding tool interference. This work can provide the theoretical basis of choosing the cutting tool path for off-axis parabolic surface and can improve the efficiency and precision of machine. Finally, some cutting simulations are implemented to prove the proposed methods.

Effects of tool overhang on selection of machining parameters and surface finish during diamond turning

In precision machining leading to nano-metric surface finish, selection of the suitable machining parameters is a critical task. To ensure the desired surface quality, one needs to optimally select the machining parametric matrix. Towards this effort, this paper adds another critical parameter in terms of tool overhang. A well-defined set of machining exercises is carried out with different tool overhangs and machining parameters. In this investigation, an attempt has been made to locate the optimum range of tool overhang with minimum tool vibrations. The interaction between tool overhang with other parameters is also thoroughly investigated. Another important focus of this study is to find out the optimum machining parameters for the situations where it is not possible to select an optimum tool overhang. One such situation occurs when a steep concave parabolic surface needs to be fabricated. In this case a large tool overhang has to be selected. Power spectral density distribution analysis of surface roughness for different tool overhangs is performed to find out significant parameters and their degree of contribution to surface roughness. Analysis of variance is also applied to ascertain statistically significant factors contributing to surface roughness. To model the surface roughness, response surface methodology is being used. The model has been verified by conducting a series of experiments and a steep concave parabolic surface is developed by following the predictions of the developed model.

Theoretical and experimental analysis of the effect of error motions on surface generation in fast tool servo machining

Abstract The fast tool servo (FTS) machining process provides an indispensable solution for machining optical microstructures with sub-micrometer form accuracy and a nanometric surface finish without the need for any subsequent post processing. The error motions in the FTS machining play an important role in the material removal process and surface generation. However, these issues have received relatively little attention. This paper presents a theoretical and experimental analysis of the effect of error motions on surface generation in FTS machining. This is accomplished by the establishment of a model-based simulation system for FTS machining, which is composed of a surface generation model, a tool path generator, and an error model. The major components of the error model include the stroke error of the FTS, the error motion of the machine slide in the feed direction, and the axial motion error of the main spindle. The form error due to the stroke error can be extracted empirically by regional analysis, the slide motion error and the axial motion error of the spindle are obtained by a kinematic model and the analysis of the profile in the circumferential direction in single point diamond turning (SPDT) of a flat surface, respectively. After incorporating the error model in the surface generation model, the model-based simulation system is capable of predicting the surface generation in FTS machining. A series of cutting tests were conducted. The predicted results were compared with the measured results, and hence the performance of the model-based simulation system was verified. The proposed research is helpful for the analysis and diagnosis of motion errors on the surface generation in the FTS machining process, and throws some light on the corresponding compensation and optimization solutions to improve the machining quality.

Effects of surface modifications on steel's machinability in single-point diamond turning

Surface modifications are employed in single-point diamond turning of steels to reduce the chemical-reactive wear of diamond tool to an acceptable degree. Different thermochemical modifications, i.e., plasma nitriding, boronising, siliconising, boronitriding and compound boronising-aluminising, are designed to bond the atoms of transition metal elements in steels with the chosen elements. The effects of various surface modifications on steels machinability are investigated mainly in terms of tool wear and workpiece surface quality. It is found that nitrogen (N) is beneficial to improve the tool life; N, silicon (Si) and aluminium (Al) may help to obtain a mirrorlike surface; B plays the negative role in steel's machinability in single-point diamond turning. Developing suitable-for-diamond-cutting materials on which nanometric surface finish can be achieved is proposed according to the role of the permeated elements.

Modeling and characterization of surface generation in fast tool servo machining of microlens arrays

A microlens array is composed of a series of microlens distributed in a regular pattern and has been used in a wide range of photonic products. Fast Tool Servo (FTS) machining is an enabling and efficient technology for fabricating high quality microlens arrays with submicrometer form accuracy and nanometric surface finish. Although there have been a number of studies on modeling and characterization of surface generation in Single Point Diamond Turning (SPTD), there is relatively little research on the modeling and characterization of surface generation in FTS machining of microlens arrays, which is radically different from SPTD and has additional process parameters. This paper therefore establishes a theoretical model for the prediction of surface generation in FTS machining of microlens arrays based on the cutting mechanism of FTS, cutting tool geometry, machining parameters, and the workpiece surface contour. A surface matching based method has been developed to characterize the surface quality of the microlens array as a whole instead of a single lens evaluation. A series of cutting experiments have been conducted, the actual results of which were found to largely agree with the predicted results. The successful development of the deterministic models and methods not only make the surface generation in FTS machining of microlens array more predictable, but also allow a better evaluation of the surface quality of the machined microlens array. It also helps to minimize or eliminate the need for conducting trial-and-error cutting experiments to optimize the machining process.

Optimized tool path generation for fast tool servo diamond turning of micro-structured surfaces

Fast tool servo (FTS) diamond turning is a promising machining process for fabricating ultra-precision micro-structured surfaces which have already gained wide application especially in the optics industry. However, most research focused on designing different types of FTSs for better performance, including bandwidth, stroke, stiffness, acceleration, accuracy, etc. There is a lack of systematical tool path generation (TPG) for FTS diamond turning, especially from the influence of TPG on surface quality point of view. This paper presents an optimized tool path generation for FTS diamond turning of micro-structured surfaces with either analytical description or non-uniform rational B-spline description. The tool path is optimized from three aspects to ensure high precision form and nanometric surface finish. Firstly, the tool path is optimized by a stable tool nose radius compensation method, which is different from the conventional one, to reduce the form error caused by the tool nose radius. Secondly, the ideal machined surface profiles are simulated to predict the ideal surface quality, with which the cutting conditions and tool geometry can be optimized to ensure nanometric surface finish and avoid tool interference. Thirdly, the influence of the control dynamics of the machine axes on machined surface quality, which is ignored in conventional machining, was considered. As the FTS dynamics dominate the influence, it is controlled to be a desired dynamics which was then used to modify the tool path to minimize the form error in machined surface. Experiments were carried out to prove the effect of TPG on surface quality and the effectiveness of the optimized TPG method.

A robust surface fitting and reconstruction algorithm for form characterization of ultra-precision freeform surfaces

Ultra-precision freeform surfaces are non-rotational symmetric surfaces possessing sub-micrometer form accuracy and nanometric surface finish. Although they can be fabricated accurately by ultra-precision machining technology, their surface quality is difficult to be characterized. Surface reconstruction is a vital task in the form characterization of ultra-precision freeform surfaces. This paper presents a robust surface fitting algorithm to reconstruct a high fidelity surface from measured discrete points while the surface smoothness can be ensured as well. A fitting threshold named confidence interval of fitting error is proposed to strike the balance between fitting accuracy and surface smoothness in the fitting process. The fitting algorithm is in two steps. In the first step, bidirectional sampling method is developed to extract a curve network from measured points cloud to construct an initial surface. In the second step, the fitting error of the initial surface is minimized to meet the prescribed fitting error threshold. A series of experimental work has been conducted and the results show that the proposed algorithm is able to provide effective means for increasing the accuracy in the form characterization of ultra-precision freeform surfaces.

Development of a novel custom micro-tool for effective cutting of a precision microgroove array on a microscope slide

This study presents a novel, economical and efficient fabrication technique for precisely generating multiple microgrooves on a microscope slide to allow for microscopic examination of urine sediment cells. This study incorporates two important phases: a precision wheel-tool array is fabricated and then the developed tool is used in fast on-line grinding of multiple microgrooves. The wheel-tool blank is made of diamond grit of 0–2 µm grade via co-deposition. Subsequently, it is trued, sliced and sharpened by means of micro wire electro discharge dressing. The finished wheel-tool is utilized on-line to grind multiple microgrooves using ‘high-speed and fast-shallow grinding’. A ductile grinding regime is established to obtain a nano-metric surface finish for the multiple microgrooves generated on the microscope slide. The depth and width of the grooves in the array are both 10 µm and a surface finish of Ra equal to 0.010 µm is simultaneously achieved. This multiple microgrooving technique can supply high-quality fast grinding in the fabrication of bio-medical devices, such as those used for stationing and counting urine sediment cells.

Ultra-precision machining of aluminium alloy surfaces for optical applications

Fabrication of optical quality freeform surfaces using ultra-precision machining technologies enables to achieve sub-micrometric form accuracy and nanometric surface finish. In this paper, ultra-precision machining of large aluminium alloy optical components has been conducted to investigate the machining performance and machined surface integrity. An ellipsoidal mirror surface and a lenticular lens array have been machined out using slow slide servo (SSS) turning, grooving and milling techniques with a single crystalline diamond tool on an ultra-precision machine centre. It was found that there are some striations on one segment of the machined ellipsoidal mirror surface. Through data density optimisation of angular increment dθ and arc length increment dc in C -axis positioning, the striation problem has been successfully solved, and surface roughness Ra of the machined ellipsoidal mirror surface has been achieved less than 10 nm. Work material microstructure and thermal stability of the machining centre significantly affected the machined lenticular lens surface quality. Compared with grooving process, milling process could produce a better surface integrity and form accuracy of large aluminium lenticular lens array.

Research on the tool alignment error in fabrication of off-axis paraboloid surfaces by using slow-tool-servo technique

Fabrication of free-form optics in good quality and high efficiency is much demanded as the widely use of free-form surfaces in modern optical systems. Slow-tool-servo technique is able to fabricate free-form optics with sub-micrometric form accuracy and nanometric surface finish in high efficiency, which has broad prospects of application. The tool alignment error is an important factor to work on the form accuracy of free-forms fabricated by slow-tool-servo technique. This paper studies the tool alignment error, presents a quantitative description of tool alignment errors at both feed and height directions by using the concept of the sensitivity function. An off-axis paraboloid workpiece was fabricated, and measure results verified the point of view in the paper. The tool alignment errors at two directions were calculated by surface figure error data to guide the alignment of cutting tool.

Study on Precision Polishing of a Mini Roller Mold

This research aimed to design and develop a polishing system for precision polishing mini roller mold to nanometer surface finish. An experimental polishing system was built in the present study to polish nickel plated specimens with various polishing compounds. The polished specimens were subsequently examined by Alfa-step, OM and SEM for surface finish, morphology and microscopic analysis respectively. The obtained surface condition and material removal rate were correlated to the polishing parameters such as spindle speed, abrasive concentration, and abrasive grit size for the improvement of the polishing effect. Mini-rollers of 5mm in diameter, 50mm in length were successfully polished to a surface roughness better than 2nm Ra in several hours without damaging the roundness and cylindricalness using abrasive of 0.3μm, 10,000rpm polishing speed and 0.5mm gap distance between polisher and the specimen. A semi-empirical model of polishing was also developed in the study for predicting the materials removal rate.

Analysis of surface generation in ultra-precision machining with a fast tool servo

The fast tool servo (FTS) is one of the emerging ultra-precision machining technologies for the fabrication of high-quality optical microstructural surfaces. The diamond tool is fixed on the FTS, which is activated back and forth by a stacked type piezoelectric actuator. Optical microstructures with sub-micrometre form accuracy and nanometric surface finish can be fabricated without the need for any subsequent post processing. However, current understanding of the cutting mechanics and the factors affecting the surface generation in FTS machining is still far from complete. Although there has been some research work conducted to fill the research gaps with respect to FTS machining, most of the previous studies have been focused on the design of FTS for better performance and modelling characteristic of FTS actuators. The study of the process factors affecting surface generation in FTS machining has received little attention. As a result, this paper aims to analyse the effects of process parameters on surface generation in FTS machining. The factors being studied include spindle speed, feed rate, and depth of cut. A series of cutting experiments was carried out under various cutting conditions and the surface quality of the workpiece in terms of the form error and form defects were studied. Based on the results of the investigation, some recommendations for optimizing surface quality in the FTS machining process are discussed.

Design of a five-axis ultra-precision micro-milling machine—UltraMill. Part 1: holistic design approach, design considerations and specifications

High-accuracy three-dimensional miniature components and microstructures are increasingly in demand in the sector of electro-optics, automotive, biotechnology, aerospace and information-technology industries. A rational approach to mechanical micro machining is to develop ultra-precision machines with small footprints. In part 1 of this two-part paper, the-state-of-the-art of ultra-precision machines with micro-machining capability is critically reviewed. The design considerations and specifications of a five-axis ultra-precision micro-milling machine—UltraMill—are discussed. Three prioritised design issues: motion accuracy, dynamic stiffness and thermal stability, formulate the holistic design approach for UltraMill. This approach has been applied to the development of key machine components and their integration so as to achieve high accuracy and nanometer surface finish.

Investigation of finish cut of microelectrodischarge milling for nanosurface finish

In this article, the machining conditions to achieve nanometric surface roughness in finish cut microelectrodischarge milling were investigated. For a constant gap voltage, the effect of feed rate and capacitance was studied on average surface roughness (Ra) and maximum peak-to-valley roughness height (Ry). Statistical models were developed using a three-level, two-factor experimental design. The developed models minimized Ra and Ry by desirability function approach. Maximum desirability was found to be more than 98%. The minimum values of Ra and Ry were 23 and 173nm, respectively, for 1.00μms−1 feed rate and 0.01nF capacitance. Verification experiments were conducted to check the accuracy of the models, where the responses were found to be very close to the predicted values. Thus, the developed models can be used to generate nanometric level surface finish, which are useful for many applications in microelectromechanical systems.